This invention relates to a method for manufacturing an essentially solvent free and dry, dimensionally stable laminar sheet of ceramic and/or glass powder particles ("frit") bound together with an organic polymer, typically in combination with a plasticizer. The polymer and plasticizer are together referred to herein as the "binder". The term "solvent" refers to a solvent for the binder, and also to a solvent for conductive paste used to provide the circuits in a multilayer or multilevel ceramic laminate ("MLC"); the term "dry" refers to solvent free and moisture free. The sheet is formed from a continuously cast slip comprising a solvent for the organic binder in combination with dispersants, surfactants or wetting agents, etc. for the frit. Though the sheet might be formed entirely of either ceramic or glass particles, the frit used for the sheet of this invention, like sheet conventionally used for the purpose herein, typically consists essentially of a major proportion by weight of a ceramic powder and a minor proportion by weight of glass powder. The sheet is, therefore, referred to herein as a "ceramic sheet" which is formed from "ceramic powder". Ingredients are chosen to give the slip desirable casting and "doctoring" (because the slip is doctored onto a belt with a blade) characteristics.
A cast sheet of ceramic powder is generally in the range from about 2 mils to about 40 mils thick and of arbitrary length and width, generally less than about 5.5 feet (about 1.65 meters). A sheet which is less than 8 mils thick is referred to as "thin" sheet to emphasize its rather fragile characteristics relative to "normal" sheet which is in the range from about 8 mils to 15 mils thick. Still thicker sheet may be made for particular purposes, not necessarily for mounting microprocessor chips, for example, mounting board for electronic components subjected to high temperatures.
After most of the solvent for the organic binder is removed from the cast sheet it is cut into pieces. Each piece of the sheet is familiarly referred to as a "card". The method for making an essentially solvent-free sheet of frit, referred to herein as a sheet of "green frit", and cutting the sheet into cards is not of particular importance but the aforedescribed method sets forth a simple, convenient, and economical way of making a "green" card conventionally.
As one might expect, drying the cast sheet to remove solvent results in shrinkage of the dimensions of the sheet. The purpose of treating the green sheet or the cards cut from the sheet is to shrink the lateral dimensions of the sheet or cards so much that upon subjecting the shrunk sheet or cards to further drying cycles, they shrink so little that they are deemed dimensionally stable.
By "dimensionally stable" is meant that a shrunk card, upon storage for aging, or drying in an oven, or both, shrinks so little that even when it is subjected to successive drying treatments, or successive periods of aging, the cumulative shrinkage is not more than 0.1 mil/inch in the direction of either lateral axis, or either diagonal. The shrinkage in the direction of the vertical axis (decrease in thickness of the card) is too small to be of significance and is ignored with respect to dimensional stability.
If not adequately stabilized despite thorough drying and aging of a green card at elevated temperature above 100.degree. C, the card thereafter continues to shrink when it is stored at room temperature, and as it progresses through subsequent processing steps required for its eventual utilization. Why an unstable card behaves in this manner is not understood. The problem is to negate such instability.
Typically the green cards are punched, screen printed with a conductive metal or metal oxide paste, dried in an oven to drive off the solvent in the paste, and laminated. The cards must be deemed dimensionally stable before they are laminated.
More particularly, a preferred slip is a mixture of alumina powders with a binder consisting essentially of an organic thermoplastic polymer and plasticizer, blended with a suitable organic solvent for each so as to form a doctorable paste. The slip is cast from a slitted nozzle and doctored onto an endless synthetic resinous belt with a blade set with a predetermined gap which determines the thickness of the cast slip. The width of the slip cast into a sheet is limited only by the distance over which a uniform thickness of the cast sheet can be reliably controlled, generally less than about 6 feet (about 2 meters). Because the cast sheet is essentially continuous and relatively narrow, it is also referred to as a "strip".
The strip travels through a drying zone where most of the solvent is removed. The strip is then cut into the cards which are relatively small pieces, usually less than 1.5 ft..times.1.5 ft. (or about 0.5 meter.times.0.5 meter), sized to lend themselves to be individually dried and stabilized sufficiently to meet the aforestated criteria for dimensional stability. The cards are then punched to provide "guide holes" which serve as through passages in which reference pins are snugly held.
After the cards are stabilized, they are punched or drilled with "vias" or "through holes" (so termed to distinguish them from guide holes) which serve to provide electrical contact between contiguous cards after they are laminated. Before they are laminated, the cards are screen printed with the conductive paste after each card is precisely positioned by reference pins through the guide holes, or otherwise provided with circuits (referred to as "metallurgy"). The conductive paste, upon being dried and sintered on a card, forms a conductor deposited on the surface of each card. The paste also fills the vias providing card-to-card electrical contact therethrough. The screen printed cards are then dried in an oven to remove solvent from the conductive paste.
Several of the screen-printed green cards are then stacked coextensively in one-to-one correspondence by inserting reference pins through the guide holes in each card, and the stacked cards are bonded under sufficient heat and pressure to form a laminate. The laminate is then cut to desired size and sintered at a temperature above 1000.degree. C. so as to result in a composite module referred to as the MLC. The early use of such MLCs in the electronics industry for the packaging of semiconductors into integrated devices and other elements is disclosed in U.S. Pat. Nos. 3,379,943 to Breedlove and 3,502,520 to Schwartz.
The foregoing problem of stabilizing a green card has drawn critical attention ever since the earliest days of the implementation of technology for making ceramic substrates used in the fabrication of electrical components with desired metallurgy.
One of the earliest solutions to the problem is disclosed in U.S. Pat. No. 3,953,562 to Hait et al. They treated a green sheet with a solvent for the organic binder for enough time to plasticize and soften but not dissolve the binder, then again dried the treated sheet before it was sintered. This solution to the problem relied on controlling the susceptibility of the green sheet to modification of its physico-chemical properties near its surface. Such modification was effected by plasticizing the green sheet, preferably by exposing the sheet soon after it was formed to solvent vapor, then restoring the sheet to its pre-plasticized state.
The criticality of maintaining dimensional stability of each card in a stack of cards prior to firing a MLC is emphasized in the foregoing '562 disclosure relating to the manufacture of MLCs, which disclosure is incorporated by reference thereto as if fully set forth herein, and in the numerous publications referred to therein.
A more recent solution to the problem was disclosed in U.S. Pat. No. 4,497,677 to Sanada et al. Their method for stabilizing a green sheet required pressing a green sheet in a press at a pressure up to 100 kg/cm2 then heating the pressed sheet to a temperature up to 250.degree. C. This approach to solving the problem, relying as it did on directly superimposed physical pressure by contact with a platen of a press, stressed the sheet's susceptibility to physical phenomena, rather than a physico-chemical phenomenon such as a plasticizing effect.
We have found, as did Sanada et al, that superposing physical phenomena upon the green sheet is more reliably effective than superposing physico-chemical phenomenon. We attribute the higher susceptibility of the green sheet to the former rather than the latter, to the fact that (in the '562 process) the green sheet has already been through the physical stage of shrinkage of the plasticized green sheet, before it was dried and re-plasticized. Again drying the re-plasticized sheet does not appear to be as well-directed as using the stimuli of physical phenomena to stabilize the sheet.
Our method, therefore, relies mainly upon superposing physical conditions in a manner which is as simple as it is easily accomplished. However, instead of using direct pressure of a press, as did the '677 method, we use contact with water under preferred temperature conditions. Whether as liquid or vapor (either or both of which are referred to herein as "moisture"), water used in our process, referred to as an "aquathermic" process, has no observable plasticizing effect on the binder and is essentially independent of pressure. The aquathermic process combines contact with moisture supplied in a preselected temperature range to accelerate the time required to dimensionally stabilize a card.